1. Field of the Invention
The present invention relates to novel colloidal dispersions of a cerium (IV) compound in an aqueous medium and to a process for the preparation thereof. This invention more especially relates to colloidal dispersions of a cerium (IV) compound in a weakly acidic aqueous medium.
In the following description of the invention, a colloidal dispersion of a cerium (IV) compound in an aqueous medium is also designated a "sol".
And by the expression "weakly acidic" is intended a pH higher than or equal to 3.5.
2. Description of the Prior Art
For numerous applications in the field of catalysis, in particular for catalysis entailing postcombustion reactions of automotive exhaust gases in catalytic converters, there currently exists a great demand for aqueous sols of a cerium compound having weak acidity.
It is described in published European Application No. 87/400,600.0 that aqueous colloidal dispersions of a cerium (IV) compound can be directly obtained by dispersing in water a cerium (IV) compound having the general formula (1): EQU Ce(OH).sub.x (NO.sub.3).sub.y, pCeO.sub.2.nH.sub.2 O (1)
wherein:
(i) x is such that x=4-y; PA1 (ii) y ranges from 0.35 to 1.5; PA1 (iii) p is greater than or equal to 0 and less than or equal to 2.0; PA1 (iv) n is greater than or equal to 0 and less than or equal to about 20. PA1 (a) destabilizing an aqueous colloidal dispersion of a cerium (IV) compound by reaction with a water-soluble salt of a monovalent acid having a pKa ranging from 2.5 to 5.0; PA1 (b) separating the resulting precipitate; and PA1 (c) subsequently redispersing said precipitate in an aqueous medium. PA1 (i) M represents an alkali metal or a quaternary ammonium radical; PA1 (ii) z ranges from 0 to 0.2; PA1 (iii) y ranges from 0 to 1.5; PA1 (iv) x is such that x=4-y+x; PA1 (v) p ranges from 0 to 2.0; PA1 (vi) n ranges from 0 to approximately 20. PA1 (i) x is such that x=4-y; PA1 (ii) y ranges from 0.35 to 1.5; PA1 (iii) p is greater than or equal to 0 and less than or equal to 2.0; PA1 (iv) n is greater than or equal to 0 and less than or equal to approximately 20. PA1 (i) [Ce.sup.(Iv) ] is the concentration, in moles/liter, of the solution of the cerium (IV) salt; PA1 (ii) V represents the volume of water with the optionally added acid; PA1 (iii) V' represents the volume of the cerium (IV) solution. PA1 (i) A represents the anion of a monovalent acid which is soluble in water and has a pK.sub.a ranging from 2.5 to 5.0; PA1 (ii) x is a number greater than or equal to 0.2 and less than or equal to 0.8; PA1 (iii) y is a number greater than or equal to 0.01 and less than or equal to 0.1.
The resulting sol is stable and has a pH ranging from 1 to 2.5.
It is possible, by basification to a pH of approximately 3, to produce a sol containing coarser colloids and having good stability.
However, this process is not suitable for producing a sol having a lower acidity.